The present invention relates to a seal ring particularly utilized for a rotating, oscillating of reciprocating portion subjected to hydraulic control such as an automatic speed change gear or a power steering mechanism of an automobile, for example.
A seal ring particularly made of polytetrafluoro ethylene resin (to be called PTFE, hereunder) having improved low friction and wear-proof properties has been widely utilized in the prior art instead of cast iron for power steering mechanisms.
In such seal ring of the prior art, a seal ring body made of resin material is fitted into a groove formed in an inner peripheral surface of a cylinder to thereby seal a portion between the cylinder and a movable member disposed in the cylinder. The seal ring body is provided with a cut portion shaped in a straight cut, bias cut, step cut or the like. Seal rings provided with the bias cut have been widely utilized because of its stable sealing properties in a wide temperature range.
However, in the prior art described, the seal ring provided with the bias cut is apt to be influenced by its thermal expansion coefficient, resulting in the following problem.
Namely, because the seal ring body made of the resin material has a thermal expansion coefficient larger than the thermal expansion coefficient of the cylinder made of a metal material such as aluminium, even when the cut portion is adjusted to substantially reduce the amount of any leakage of oil, for example, at an ambient temperature, the size of the gap of the abutment portion of the cut ends of the seal ring body increases at a temperature higher than at the ambient temperature, resulting in the increasing of the leakage amount. On the other hand, even when the abutment portion is adjusted to substantially reduce the leakage at a high temperature, the size of the gap at the abutment portion also increases because the amount of shrinkage in the seal ring body is higher than that of the cylinder. This shrinkage results in the increasing of the leakage amount at the ambient temperature.
Furthermore, when the amount of leakage at the ambient temperature is reduced, the seal ring body has an interference fit with the cylinder at the high temperature, such that the minimum pressure, (i.e. minimum actuating pressure) of the seal ring body movable in the groove is adversely increased.
The above described problems result from the dimensional change of the gap at the abutment portion of the seal ring body due to the temperature change. In other words, the thermal expansion coefficient of the seal ring body is larger than that of the cylinder. In order to effectively reduce the dimensional change of the gap, it may be possible to reduce the range of the common difference of the outer diameter of the seal ring body (for example, to about 0.04 mm with respect to .phi. of 56 mm), but this working is troublesome and degraded products can often result.